1. Field of the Invention
This invention relates to a ceramic heater, a glow plug, and a ceramic heater manufacturing method.
2. Description of the Related Art
JP-A-2003-56848 discloses a glow plug including a cylindrical metal shell, a columnar ceramic heater provided inside the tip end side of the metal shell and having a heating part at its tip end, and a center pole provided in the rear end side of the metal shell.
The ceramic heater of JP-A-2003-56848 has a cylindrical substrate made of a silicon nitride insulating material, a resistor buried in one end side of the substrate, a pair of lead parts buried in the substrate, the lead parts being connected to the resistor and extending to an opposite end side of the substrate, and electrode parts exposed at the opposite end surface of the substrate and connected to the lead parts. The ceramic heater of JP-A-2003-56848 has a metal cylinder and a metal ring on its outer periphery; one electrode part is electrically connected to the metal shell through the metal cylinder and the other electrode part is electrically connected to the center pole through the metal ring.
The resistor, the lead parts, and the electrode parts constitute a fired body made of an insulting material of silicon nitride and tungsten carbide (WC), for example. In this case, the lead parts and the electrode parts each has a larger WC content than that of the resistor, and accordingly has a higher electrical conductivity than the resistor.
The metal shell of the glow plug is fixed to a cylinder head of a diesel engine and the center pole is connected to a battery. In the ceramic heater, a voltage is applied between the metal shell and the center pole, whereby the resistor is energized through the electrode part and the lead part, to generate heat. Accordingly, the glow plug is utilized when starting the diesel engine and while idling.
This type of ceramic heater is manufactured as follows: First, for example, a resistor green part which is to become a resistor is injection-molded with a material having a decreased WC content and then a lead green part which is to become a lead part and an electrode part are injection-molded with a material having an increased WC content to provide a one-piece molded green body including the resistor green part and the lead green part.
The molded green body thus prepared is fitted into a substrate green body which is to become a substrate, and is pressed to produce a green body assembly. Then, the green body assembly is subjected to a debinder treatment and fired at 1700° C. or higher, for example, in the vicinity of about 1800° C. with a hot press, etc. The fired body thus obtained is ground to produce a ceramic heater.
As compared with a ceramic heater having a lead part and an electrode part made of metal wires, the ceramic heater having the resistor, the lead part, and the electrode part sintered from materials together with the substrate green body eliminates the need for a metal lead wire manufacturing process of bending and cutting wires made of W, W—Re alloy, etc. In addition, the lead parts and the electrode parts can be molded together with the resistor. Thus, as compared with the former ceramic heater, the latter has the advantage of high mass productivity. Thus, for example, the ceramic heater as described above enables a thin glow plug to be manufactured at low cost.
3. Problems to be Solved by the Invention
However, there is a strong need to enhance the long term durability of this kind of ceramic heater. For example, there is a need to suppress break-resistant strength degradation of the ceramic heater with use as a measure of durability. Particularly, recent high performance diesel engines require a small and thin glow plug. In such a glow plug, a ceramic heater having a substrate measuring about 3.5 mm or less in diameter is required, and the problem of long term durability becomes evident.